INITIATION OF IMPULSES AT RECEPTORS 



'33 



100 r 

 % 

 90 



70 

 60 

 SO 



■10 

 )0 

 20 - 

 10 



F19.5 



Stiinulus scrength 



10 

 1 



Fig 6 



o O 



oo 



10 

 2 



FIG. 5. Receptor potential amplitude in relation to the displacement of the mechanical stimulus 

 with velocity constant in a Pacinian corpuscle. Abscissa: stimulus strength in arbitrary units. Ordi- 

 nate: receptor potential amplitude as percentage of maximum. O same points as •, but stimulus 

 strength scale expanded five times. [From Gray & Sato (37).^ 



FIG. 6. Receptor potential rate of rise in relation to the displacement of the mechanical stimulus 

 with velocity constant in a Pacinian corpuscle. Abscissa: stimulus strength in arbitrary units. Ordi- 

 nate: receptor potential rate of rise as percentages of maximum amplitude per msec. O same points 

 as •, but stimulus strength scale expanded five times. [From Gray & Sato (37).] 



tained from a Pacinian corpuscle (37), but a siinilar 

 relationship has been observed in the muscle spindle 

 of the frog (58}. The value of the receptor potential 

 reaches a constant level with large displacements; this 

 is not conclusively proved by the published data, 

 partly because of the limits to the size of stimulus used 

 and partly, in the Pacinian corpuscle experiments, 

 because the biggest stimuli introduced artifacts that 

 tended to sum with the response. The question of the 

 absolute amplitude of this ma.ximum is considered in 

 a later section. 



The rate of rise of the potential is also related 

 to the size of the exciting displacement (37). This is 

 shown in figure 6. There is a change in the slope of 

 this graph at that level of stimulus strength above 

 which the amplitude increase is limited, but even 

 above this point the rate of rise of the potential con- 

 tinues to increase with stimulus strength. That this is 

 a genuine effect is supported h\ the fact that the time 

 of rise of the potential continues to shorten over this 

 range of stimuli. Since the recorded potential is a 

 result of a potential change across the terminal mem- 

 brane (whether or not the potential is actively gen- 

 erated at this site), the rate of rise of the receptor 

 potential will reflect the rate at which current flows 

 in to the capacity of this membrane. In other words 

 these results suggest that the current across the mem- 

 brane of the nerve fiber terminal continues to increase 



as the stimulus increases even though the peak po- 

 tential has reached a maximum \alue. 



The amplitude of certain receptor potentials, for 

 example those of the Pacinian corpuscle and the 

 early phase of that of the frog's muscle spindle (fig. 

 \A), is also dependent on the velocity of the displace- 

 ment. Indirect evidence shows that this is also true of 

 other receptors responding to other forms of energy, 

 for example thermal receptors (46). Figure 7 illus- 

 trates the change in relati\e ainplitude of the receptor 

 potential that accompanies change in the velocity of 

 the mechanical stimulus, the amplitude of the stimu- 

 lus ijeing kept constant. It is immediately clear that 

 the amplitude of the receptor potential, while inde- 

 pendent of velocity at high values, is over a certain 

 range closely related to the velocity of the stimulus. 

 The 'angle' of this curve occurs, for the Pacinian 

 corpuscle, at a compression velocity of about i mm 

 per sec. (i.e. about 5 thresholds per msec). This 

 means that many physiological stimuli may be ex- 

 pected to lie within the velocity-sensitive range. 



The time course of the receptor potential is in most 

 instances dependent on the properties of the stimulus. 

 It has already been pointed out that the rate of rise 

 of the potential \aries with stimulus amplitude and 

 this change in the rate of rise of the potential change 

 is accompanied by a change in the time of rise (37, 

 58). The rate of rise of the potential change may also 

 be affected by the \elocity (or comparable time 



